In recent years, reduction of amount of waste processing solutions is strongly desired in the medical field from the standpoint of environmental protection and space savings. Techniques relating to photosensitive heat-developable photographic materials for use in medical diagnosis and photomechanical processes are required which enable efficient exposure by a laser image setter or laser imager and formation of a clear black image having high resolution and sharpness. The photosensitive heat-developable photographic material can provide users with a simple and non-polluting heat development processing system that eliminates the use of solution-type processing chemicals.
Although the same is required also in the field of general image-forming materials, the image for medical diagnosis in particular must be finely drawn and therefore, high image quality with excellent sharpness and graininess is needed. Moreover, in view of diagnostic convenience, an image of cold black tone is preferred. At present, various hard copy systems using a pigment or a dye are commercially available as a general image-forming system, such as ink jet printer and electrophotography, however, these are not a satisfactory output system for the medical-use image.
On the other hand, thermal image forming systems using an organic silver salt are described, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, B. Shely, Thermally Processed Silver Systems, and Sturge, V. Walworth and A. Shepp (compilers), Imaging Processes and Materials, 8th ed., page 2, Neblette (1996). In particular, heat-developable photosensitive materials generally have a photosensitive layer comprising a binder matrix having dispersed therein a catalytic amount of a photocatalyst (for example, silver halide), a reducing agent, a reducible silver salt (for example, organic silver salt) and if desired, a color toner for controlling the silver tone. The heat-developable photosensitive material after image exposure is heated at a high temperature (for example, 80° C. or more) to bring about an oxidation-reduction reaction between the silver halide or reducible silver salt (acting as an oxidizing agent) and the reducing agent and thereby form a black silver image. The oxidation-reduction reaction is accelerated by the catalytic action of a silver halide latent image generated upon exposure. Therefore, the black silver image is formed in the exposed area. This is disclosed in many publications including U.S. Pat. No. 2,910,377 and JP-B-43-4924 (the term “JP-B” as used herein means an “examined Japanese patent publication”). As a medical image forming system using a heat-developable photosensitive material, “FM-DP L” (Fuji Medical Dry Imager) is put on the market.
For the production of a thermal image forming system using an organic silver salt, a method of producing the system by coating a solvent, and a method of producing the system by coating and drying a coating solution containing, as a main binder, an aqueous dispersion of fine polymer particles are known. The latter method needs only a simple production equipment and is suited for mass production, because a step for collecting a solvent is unnecessary.
In either the coating method using a solvent or the aqueous coating method using mainly water as the solvent, the coating of a heat-developable photosensitive material is difficult as compared with conventional photosensitive materials using gelatin as a main binder. In particular, high-speed coating causes generation of streaks or unevenness. In order to improve the productivity and profitability, improvement is demanded in the coatability.
Furthermore, use of an aqueous latex as a binder has a problem in that on touching with a hand wetted with sweat or oil, a fingerprint attaches and this causes discoloration in aging.